The invention is concerned with glasses having a high conductivity for sodium ions. It is especially concerned with glasses of this nature having other characteristics that adapt them to use as solid electrolytes in electrochemical cells.
An application of particular interest is the alkali metal-sulfur battery as exemplified in U.S. Pat. Nos. 3,404,035 and 3,476,602. A variety of designs and materials has been proposed for this type battery. Thus far, however, the sodium-sulfur version appears to have generated major interest.
Typically, the sodium-sulfur battery utilizes a liquid sodium anode, a liquid mixture of sulfur and sodium sulfide (e.g., a sodium polysulfide) as a cathode, and a sodium ion conductive glass as an electrolyte-separator or membrane. This membrane separates the anodic and cathodic liquids, and is permeable to sodium ions.
The membrane may take various shapes that provide suitable sodium ion transfer, while keeping the liquid anode and cathode materials separated. Suitable membrane shapes include flat plates, corrugated sheets, spirals, and hollow fibers. Maximum efficiency and energy density have been secured by making the membrane (electrolyte-separator) very thin in cross-section.
Both ceramic and vitreous materials have been proposed for use in producing electrolyte separators. Beta-alumina, as such or modified, has been used quite widely. In the field of glasses, it has been proposed to use silicate, borate, borosilicate, boroaluminate, and borophosphate systems. All of these have failed to prove adequate in one or more respects.
A glass membrane, useful for separating the anodic and cathodic liquids in a sodium-sulfur battery, must satisfy two very basic requirements. First and foremost, the glass must provide a sufficiently high sodium ion conductivity at a temperature of 300.degree. C., a typical operating temperature for such a battery. For this reason, the glass must have a resistivity of 10.sup.5 ohm-cm or less, corresponding to a sufficiently high conductivity, at that temperature.
In addition to the glass resistivity being low, it must remain relatively stable during the life of a battery in order to maintain a stable conductivity. To this end, the glass must be resistant to corrosion, and non-reactive with the contacting liquids, during the operational life of the battery.
Many of the glasses previously proposed in the literature have sufficiently low resistivities, but only a few provide any reasonable degree of corrosion stability. Among these are the sodium borate glasses described in U.S. Pat. No. 3,829,331 (Tsang) and the aluminoborate glasses disclosed in U.S. Pat. No. 4,190,500 (Booth).
In addition to the two basic considerations of electrical conductivity and stability, there are several other factors to be considered in selecting or developing a glass for use as a membrane in a sodium-sulfur battery. These include: (1) viscosity-temperature characteristics, (2) resistance to devitrification, (3) resistance to atmospheric attack, and (4) structural stability at an elevated temperature. Their relative importance depends on the design, mode of operation and process of production for the battery.
The viscosity-temperature characteristics of a glass must be so controlled as to provide an annealing point that is compatible with formation of an effective seal with a sealing glass. This requirement is discussed in detail in my pending application Ser. No. 195,377 filed Oct. 9, 1980 now U.S. Pat. No. 4,311,772. Thus, the membrane glass may be required to withstand the elevated temperature necessary to melt a sealing glass. Alternatively, if an ion exchange sealing process is employed, it must withstand the temperature at which exchange is effected. Both the sodium borate and sodium aluminoborate glasses tend to be marginally useful in this respect.
The membrane glass must resist devitrification sufficiently to permit the initial forming operation, as well as avoid any crystallization during sealing. The development of uncontrolled crystallization (devitrification) creates random stresses that tend to weaken the glass and cause fracture. By way of example, a glass that does not devitrify, when cooled at a 2.degree./minute rate from its melting temperature, is usually considered adequate for drawing of tubing such as used in battery electrolyte separators. Aluminoborate glasses, that have low resistivity due to a high soda content, tend to devitrify easily, and thus fail to meet this requirement.
Resistance to atmospheric corrosion or attack is vital in order to prevent surface deterioration which results in loss of mechanical strength as well as in variations of electrical characteristics. The usual problem is moisture attack. Both sodium borates and sodium aluminoborates tend to have poor weathering characteristics, thus failing to meet this requirement.
As noted earlier, a sodium-sulfur battery operates at about 300.degree. C. The membrane must remain structurally and chemically stable at this temperature. Structural change can lead to viscous deformation, or to stresses which result in fracture and change in conductivity. Chemical change may be a result of reaction with water vapor or impurities in the battery chemicals. In either case, glass properties may vary unpredictably. Both sodium borate and sodium aluminoborate glasses are deficient in this respect, but the former are particularly susceptible.